176 



"The sun's rays," says Tyndall*, "striking upon the unpro- 

 tected surface of the glacier, enter the ice to a considerable depth ; 

 and the consequence is that the ice near the surface of the glacier is 

 always disintegrated, being cut up into minute fissures and cavities 

 filled with water and air, which, for reasons already assigned, cause 

 the glacier when it is clean to appear white and opaque. The ice 

 under the moraines, on the contrary, is usually dark and transparent. 

 I have sometimes seen it as black as pitch, the blackness being a 

 proof of its great transparency, which prevents the reflexion of light 

 from its interior. The ice under the moraines cannot be assailed in 

 its depths by the solar heat, because this heat becomes obscure be- 

 fore it reaches the ice, and as such it lacks the power of penetrating 

 the substance. It is also communicated in great part by way of con- 

 tact instead of by radiation. A thin film at the surface of the mo- 

 raine ice engages all the heat that acts upon it, its deeper portions 

 remaining transparent and intact." 



It matters not to the argument how little below freezing the tem- 

 perature of a glacier may be. So long as the ice exists in a solid state 

 and is capable of being penetrated by the solar heat, it cannot but 

 dilate and contract. Its central portions, lying folded in ice 100 feet 

 thick above and below, may well, however, be conceived to retain 

 some of the cold of the region from which they have descended. The 

 observations of Agassiz on the temperature of the Aar Glacier are not to 

 be relied upon, because the access of damp external air to the borings 

 in which they were made, and of water percolating the disintegrated 

 ice of the surface, was not effectually stopped. The included 

 thermometers could not but under these circumstances show zero, 

 although the temperature of the surrounding ice was below it. For 

 the water freezing on the walls of the boring, the latent heat thereby 

 given out, would raise the temperature of the air about the bulb to the 

 freezing-point, and this water being continually renewed, the quick- 

 silver would always be kept at that point. 



That glacier-ice possesses no such properties of viscidity or com- 

 pressibility as would cause it to descend by its weight along such 

 slopes as those on which some glaciers descend may be shown thus. 

 Let the Mer de Glace be conceived to be cut up by vertical sections 

 at right angles to one another, into blocks, whose bases are large 

 enough to prevent them toppling over; and let these blocks be 

 * Glaciers of the Alps, p. 294. 



